A net terminal which enables a transition to be made from a two-wire connection to a four-wire connection is known from our specification U.S. Pat. No. 4,539,443.
An exchange terminal circuit is normally located in a line terminal of a telephone exchange and has two input terminals for connection of the two-wire line. The terminal circuit must be capable of fulfilling many requirements. For instance, the circuit input terminals must be balanced against a reference potential, normally earth, since so-called longitudinal currents will otherwise occur over the line. Furthermore, the exchange terminal circuit shall exhibit a high degree of echo attenuation. Effective attenuation distortion and basic attenuation within a prevailing frequency band, i.e. with either the speech frequency band or the bit frequency band of the digital signals, shall be small. Another requirement is that so-called loop attenuation shall be low between transmitting and receiving equipment on the subscriber side or the telephone exchange side or the like.
In the case of a net terminal, the terminal shall enable connection of a supply voltage source which delivers current to the subscriber equipment located at the other end of the line, through the circuit input terminals. This current is normally used for line signalling purposes, in conjunction with telephony.
The known subscriber net terminal is intended for the transmission of speech signals within the frequency range of about 300 Hz to about 3.5 kHz, and for the transmission of line signalling signals, which, as is known, are of the direct current type. Problems occur, however, when the subscriber line is used for the transmission of digital signals, which signals are transmitted at much higher frequencies than the speech frequencies. The digital signals are attenuated much more strongly than the speech signals, and consequently it is necessary to transmit the digital signals at a higher power level. This in turn places a greater demand on good earth balance.
The purpose of the overvoltage and overcurrent protective circuit is to protect expensive equipment connected to the output side of the terminal circuit or net terminal against overcurrents and overvoltages caused, for instance, by lightning voltages on the subscriber line, interfering electromagnetic fields, unintentional connection of the line to the electrical power network, e.g. 220 V a.c. current, as a result of fire for instance, or as a result of clumsiness, or as a result of unsuitable positioning of the line in the close proximity of a power cable, such that the line and cable are brought into electrical contact with one another through the influence of stormy weather, animals or in some other fashion.
The overcurrent and overvoltage protective circuit shall not influence the aforesaid electrical properties of the terminal circuit.
Thus, when the line is earthed or short-circuited, it is necessary to limit the current delivered to equipment in order to ensure that the equipment will not be damaged. Digital exchange equipment connected to the line will normally have an operating voltage of 5 V and is dimensioned to withstand a maximum voltage of about 5 volts without being destroyed. The overvoltage generated on the line by lightning may, for instance, be in the order of 1500 volts.
In present day systems, large glow valves, or cold cathode tubes, are connected between each wire of the subscriber line and earth on the input side of a telephone exchange as a primary means of protection. These glow valves are sluggish and have a reaction time of about 1 ms. The maximum voltage of a lightning pulse is manifest after about 10 microseconds. Furthermore, the flow valve is triggered at different moments in time, resulting in the occurrence of a transversal residual voltage over the two wires of the line.
Fire must never occur in an overvoltage and overcurrent protective circuit. In the case of a known overvoltage and overcurrent protector of the kind illustrated in the accompanying FIG. 1, the overvoltage is restricted with the aid of two wire-wound resistors. If the overvoltage is of long duration and high, there is a danger that these wire resistors may begin to glow and therewith present a fire risk. These wire-wound resistors must be matched or paired with one another in order to prevent the overvoltage protector from influencing the earth balance of the terminal circuit. The so-called component nominal voltage of the resistors, i.e. their durability to voltage, must be high in order to be able to resist high overvoltages. Furthermore, the resistors must be physically large, in order to withstand high powers.
The known protective circuit also includes a line transformer. When the current passing through the transformer is high, there is a danger that the foil on the circuit cord or board on which the overcurrent and overvoltage protector is mounted will begin to burn.
It is known to use temperature responsive resistors, so-called PTC-resistors, as current limiting protectors. The drawback with these resistors, however, is that their component nominal voltage is limited, and consequently electric sparking is likely to occur in a component should the component be subjected to a high voltage. The positioning of these PTC-resistors is also critical. If the current passing through a PTC-resistor is high, the resistor will begin to melt and the molten material is liable to ignite the underlying foil board or card. When PTC-resistors are subjected to high currents and voltages, they also tend to crack as a result of temperature gradients occurring in the resistor mass, whereupon the protective facility no longer exists.
The use of zener-diodes as a means for protecting against overvoltages is known to the art. Such zener-diodes shall have the steepest possible characteristic curve. This creates dynamic problems, however, since many harmonics (overtones), harmonic distortions and intermodulation products occur, which have a negative influence on the quality of signal transmission.
The earth balance requirement implies, among other things, that the voltage imbalance in the protective circuit may reach at maximum 60 decibels. When seen against a line impedance of about 600 ohms, this means that the resistance in the earth symmetrical circuits of the line voltage protector may mutually differ by about 0.1 ohm at most. Line-running is therefore critical.